Battelle-Columbus Laboratories has conducted extensive research on air filtration systems and especially on electrostatic-assisted systems. In the course of this work, corona-type ionizers have been used to charge air-borne dust, and the charge on airborne dust has been measured and studied in various systems. A relationship was noted between the charge detected in the air downstream from an ionizer and the amount of dust in the airstream, even in relatively clean air.
Electrical coronas commonly are used for charging particles in various types of electrostatic systems for controlling particles in air streams. Coronas produce large numbers of unipolar ions which impart charge to airborne particles by diffusion and bombardment processes. Typically, ion concentrations of 10.sup.7 to 10.sup.9 ions per cc can be produced in suitable charging configurations, and the ions have mobilities of about 1.8 to 2.0 cm/sec/volt/cm. These high charge concentrations generally are restricted to the electrical field in the vicinity of the ionizer where the corona is produced.
However, significant charge concentration often can be measured downstream from the ionizers also. Various investigators have noted charge concentrations in outlets from electrostatic precipitators, and certain types of static eliminators produce "charged air". Frequently, this charge is attributed to ions produced by coronas. However, the mobility of gas ions is very high; and, at the usual gas velocities in air cleaners, few if any ions should be blown out of the electrical field where they are generated. For example, if a potential difference of 10 kv is applied between two flat electrodes spaced 1 cm apart, the electrical field strength in the gap between the electrodes is 10 kv/cm. An ion in this field will have a velocity of 2 cm/sec/volt/cm.times.10.times.10.sup.3 volt/cm or 2.times.10.sup.4 cm/sec, and the ion will cross the 1 cm gap in 5.times.10.sup.-5 sec. If the air flow through the gap is 100 fpm or 50 cm/sec, a pair of flat electrodes only 2.5.times.10.sup.-3 cm long would effectively trap all of the ions generated in the gap.
In U.S. Pat. No. 2,262,370, Penney, dust concentration is measured as a function of the current in an ionizing circuit. This is an active type of detection. Penney's apparatus may include an upstream ionizer to increase the sensitivity, but it is optional. Whether with or without the extra ionizer, Penney's active detection apparatus is much less sensitive than the apparatus of the present invention, wherein the detection is of the passive type.
In accordance with normal usage, an "active" detection circuit is one in which energy is supplied and the condition to be detected can affect the rate of consumption of the energy, while a "passive" detection circuit is one in which the condition to be detected itself furnishes a detectable amount of energy. A typical active detector provides current and measures fluctuations in the current. A typical passive detector operates without current until air flow carries a current of charged particles into the detector.
Typical apparatus according to the present invention for detecting the presence of more than a negligible concentration of dust or other particles in a moving stream of gas comprises first and second electrically conductive members spaced apart in a first region of the stream, means for providing thereto electrical potentials sufficiently different to provide a potential gradient of at least about 30 kilovolts per centimeter in a limited region adjacent to the surface of one of the conductive members, to provide a corona and thus to produce ions that charge a substantial proportion of any particles in the gas passing through the first region; and passive means in a second region of the stream that is downstream from the first region, for detecting the presence of a detectable concentration of charged particles in the gas while present in the second region.
The passive means comprises third and fourth electrically conductive members spaced apart in the second region, means for providing thereto electrical potentials of such difference as to provide a potential gradient of substantially less than about 30 kilovolts per centimeter therebetween, and means for detecting any measurable current flowing to the third or the fourth member. Typically either the third or the fourth conductive member, but not both, comprises an extension of either the first or the second conductive member. Commonly the gas is air.
Typically the first and second conductive members are located substantially coaxially with the stream in the first region. The first conductive member typically comprises a hollow and substantially cylindrical conductor surrounding the sides of the first region and open at both ends, and the second conductive member comprises a relatively thin conductor positioned inside the first conductive member and located approximately along the axis of the first conductive member. The third and fourth conductive members are typically located substantially coaxially with the stream in the second region. Typically the third conductive member comprises a hollow and substantially cylindrical conductor surrounding the sides of the second region and open at both ends, and the fourth conductive member comprises a relatively thin conductor positioned inside the third conductive member and located approximately along the axis of the third conductive member.
Where the first conductive member comprises a hollow and substantially cylindrical conductor surrounding the sides of the first region and open at both ends, the second conductive member comprises a relatively thin conductor located approximately along the axis of the first conductive member, the third conductive member comprises a hollow and substantially cylindrical conductor surrounding the sides of the second region and open at both ends, and the fourth conductive member comprises a relatively thin conductor located approximately along the axis of the third conductive member. The third conductive member typically comprises an extension of the first conductive member. Alternatively, the fourth conductive member may comprise an extension of the second member, in which case the third conductive member is insulated from the first conductive member (and typically from the rest of the flow system).
The apparatus typically comprises also filter means upstream from the first region for removing dust and other particles from the stream of gas, a gas receiving device downstream from the second region, and means such as a servo mechanism responsive to the current detecting means for avoiding damage to the receiving device whenever the current detecting means indicates the presence of a predetermined concentration of charged particles in the second region. Where the gas receiving device is an engine, the current responsive means typically comprises means such as a valve for blocking the flow of gas between the second region and the engine. The current responsive means typically comprises also means for stopping the engine. The current responsive means may comprise also means for providing a warning signal such as a visible or audible alarm.